1. Field of the Invention
The present invention relates to high Cr Ferritic/Martensitic steels having improved creep resistance for in-core component materials in a nuclear reactor and a preparation method thereof.
2. Description of the Related Art
The sodium-cooled fast reactor (SFR) uses a fast neutron, and has nuclear fuel breeding characteristic. Accordingly, since the early stage of nuclear power industry, SFR has been continuously developed mainly for efficient use of uranium resources. Recently, as reflected in the Generation IV reactor (Gen IV) development program, the sodium-cooled fast reactor has regained the spotlight for recycling of used nuclear fuels and transmutation of long-lived radionuclide wastes.
Nuclear fuel is an essential element of sodium-cooled fast reactor in which processing such as nuclear fission for energy generation, fuel breeding from nuclear material or transmutation of nuclear waste is performed. Therefore, the stability of nuclear fuel in which radioactive nuclear fission products are contained is directly related to the stability of nuclear reactor.
Since a nuclear fuel cladding tube seals fuel slug and prevents radioactive materials from leaking, the nuclear fuel cladding tube is the most important nuclear fuel component which is directly related to the safety of nuclear fuel and a nuclear reactor. The nuclear fuel cladding tube of SFR is designed to use in severe conditions of high temperature and high neutron irradiation. Therefore, a cladding tube having excellent creep resistance at high temperature and a constant ductility while having a low swelling until high neutron irradiation should be developed. In order to realize this, the development of a new material having high temperature/irradiation resistance under conditions of coolant at high temperature and high neutron irradiation, and good compatibility with liquid sodium.
Thus, high Cr Ferritic/Martensitic Steel (FMS) which has superior properties at a high temperature has drawn wide attention as a candidate material for major core components in Generation IV reactor and nuclear fusion reactor.
The FM steel including 8 to 12% by weight of chromium has been used as a material for the in-core components of the fast breeder reactor which uses fast neutrons, including a nuclear fuel cladding tube, a duct which wraps the nuclear fuel cladding tube, since the 1970 because FMS has the superior thermal properties and irradiation swelling resistance, compared to austenitic stainless steels (e.g., SS316, SS304).
The high Cr FM steel may be largely classified into 9Cr-1Mo (ASME T9) series and 12Cr (AISI 410) series, and the course in which the high Cr FM steel has been modified is shown in FIG. 1. As shown in FIG. 1, as the 9Cr-1Mo series, 9Cr-2Mo (HCM 9), 9Cr-2MoVNb (EM12), and 9Cr-1MoVNb (Tempaloy F-9) having a creep rupture strength of about 60 MPa at 600° C. for 105 hours were developed, and later 9Cr—MoVNb (ASME T91) having a creep rupture strength of about 100 MPa was developed. In addition, Sumitomo Corp. of Japan developed 9Cr-0.5Mo-1.8 WVNb (ASME T92) having a creep rupture strength of about 130 MPa by reducing Mo element from ASME T91 and adding W, and NF12 (11Cr—WCo—NiVNb) alloy having a creep rupture strength of about 150 MPa was also developed.
12Cr-1Mo—VW (HT9), 12Cr-1Mo-1WVNb (HCM12), and 11Cr-0.4Mo-2WVNbCu (ASME T122) were developed as the 12Cr series, and 11Cr—WCo—VNb (SAVE12) steel having a creep rupture strength of about 150 MPa was developed.
As shown in FIG. 1, it was determined that in the development process of high Cr Ferritic/Martensitic Steel (FMS), a steel to which Co was added as an alloy element had an excellent creep rupture strength, and a high Cr Ferritic/Martensitic Steel (FMS), to which Co was added to have an excellent heat resistance and creep rupture strength, was disclosed in EP 0806490B1.
However, as disclosed in EP 0806490B1, when a Ferritic/Martensitic Steel (FMS), to which Co components are added, is used, a safety issue for workers working in sealed nuclear power plants emerges, and thus the steel is not appropriate for nuclear energy, in particular, as a material related to nuclear reactors.
In the mid 1980s, material development program of nuclear fusion reactor has begun to develop in earnest, and the concept of reduced-activation steel was introduced. In such a circumstance, studies of low radioactive FM steel (RAFMS) were actively conducted, starting with the material such as FM steel of ASTM GR.91 alloy (main components: 9% Cr-1% Mo-0.20% V-0.08% Nb), which is well known as modified 9Cr-1 Mo steel. The low radioactive FM steel has limitations in terms of the alloy elements added to reduce long-lived high level radioactive material generated by fast neutron irradiation. That is, the addition of molybdenum, niobium, nickel, copper, and nitrogen to low radioactive FM steel was strictly limited. Instead, adding tungsten and tantalum to low radioactive FM steel was suggested. Also, an alloy with 7 to 9% reduced chromium is preferred as a way of inhibiting the generation of δ-ferrite phase which has bad influence on impact properties without increasing addition of carbon or manganese which is an α-phase stabilizing element. With these series of studies, F82H alloy (main components: 8% Cr-2.0% W-0.25% V-0.04% Ta) and JLF-1 alloy (main components: 9% Cr-2.0% W-0.25% V-0.05% Ta-0.02% Ti) from Japan, EUROFER-97 alloy (main components: 9% Cr-1.1% W-0.20% V-0.12% Ta-0.01% Ti) from Europe, and ORNL 9Cr-2WVTa (main components: 9% Cr-2.0% W-0.25% V-0.07% Ta) from US have been developed.
However, since a SFR nuclear cladding tube is used under severe conditions such as high temperature and irradiation of fast neutrons, it is still necessary to develop a high Cr Ferritic/Martensitic steel having improved creep resistance.
Thus, the present inventors have studied to develop high Cr Ferritic/Martensitic steels having improved creep resistance at high temperatures, and developed a high Cr Ferritic/Martensitic steel exhibiting excellent creep resistance by optimizing the composition of alloying elements of niobium, tantalum, tungsten, nitrogen, boron, carbon, and the like, thereby completing the present invention.